1. Field of the Invention
The present invention relates to a silica sol capable of providing a porous film excellent in dielectric properties, adhesion, uniformity of thin film, and mechanical strength, a film forming composition, a method for forming a porous film, a porous film formed thereby, and a semiconductor device having the porous film therein.
2. Description of the Related Art
In the fabrication of semiconductor integrated circuits, as their integration degree becomes higher, an increase in interconnect delay time due to an increase in interconnect capacitance, which is a parasitic capacitance between metal interconnects, prevents their performance enhancement. The interconnect delay time is called an RC delay which is in proportion to the product of electric resistance of metal interconnects and the static capacitance between interconnects. Reduction in the resistance of metal interconnects or reduction in the capacitance between interconnects is necessary for reducing this interconnect delay time. The reduction in the resistance of an interconnect metal or the interconnect capacitance can prevent even a highly integrated semiconductor device from causing an interconnect delay, which enables miniaturization and high speed operation of the semiconductor device and moreover, minimization of the power consumption.
In order to reduce the resistance of metal interconnects, copper interconnects have recently replaced conventional aluminum interconnects.
Use of copper interconnects alone, however, has limits in accomplishing performance enhancement so that reduction in the interconnect capacitance is an urgent necessity for further performance enhancement of semiconductor devices.
One method for reducing interconnect capacitance may be to reduce the dielectric constant of an interlayer insulating film disposed between metal interconnects. It is the common practice to prepare a material having a dielectric constant of 2.5 or less by introducing pores therein to make it porous.
When an interlayer insulating film is made porous, however, reduction in mechanical strength and adsorption of moisture tend to deteriorate the film so that reduction in dielectric constant (k) by introduction of pores in the film and maintenance of sufficient mechanical strength and hydrophobicity are big challenges to be solved.
For satisfying both introduction of pores and sufficient mechanical strength, proposed is a method of introducing zeolite or zeolite-like structure, as ultimately hard particles, into a film to raise its strength or forming crystals to reduce remaining silanol groups, thereby maintaining sufficient hydrophobicity. For example, California University/USA has proposed a method for forming a zeolite film (silica zeolite film having an MFI crystal structure) on a semiconductor substrate by using a suspension obtained by separating and removing particles of a relatively large particle size from zeolite fine particles obtained by hydrolysis, in the presence of tetrapropylammonium hydroxide (TPAOH), of tetraethylorthosilicate (TEOS) dissolved in ethyl alcohol (refer to, for example, US Patent Application Publication No. 2002/0060364 A1, Advanced Material, 13, No. 19, 1453-1466 (2001)). Although the zeolite film obtained by the above-described method has a Young's modulus of from 16 to 18 GPa, it cannot be suited for practical use because due to high hygroscopicity of the film, it absorbs atmospheric moisture and drastically raises its dielectric constant (for example, it increases from 2.3 to 3.9). There is therefore proposed a method of keeping a dielectric constant of the film to from 2.1 to 2.3 by silane treatment for making the film surface hydrophobic.
There is also proposed a method for heightening the strength by using zeolite particles/zeolite-like particles and an alkoxysilane hydrolysate in combination (refer to, for example, Japanese Patent Provisional Publication No. 2004-153147). In this method, zeolite particles or zeolite-like particles are formed first and they are mixed with the alkoxysilane hydrolysate, optionally followed by a ripening reaction. The method for forming crystalline zeolite thus requires such a complex operation.
A synthesis method of zeolite having a low impurity content and suitable for use in semiconductor devices as described above is very cumbersome. There are many attempts to obtain a low-dielectric-constant film by using a silicon oxide-based polymer which is advantageous to an industrial process application compared to zeolite. For example, in Japanese Patent Provisional Publication No. 2004-149714, recommended is a method for improving a pore density of a film by using a large amount of tetrapropylammonium hydroxide acting as a structure directing agent upon synthesis of zeolite to partially form a zeolite-like structure, thereby forming zeolite-like micropores in the film during film formation.
The film strength itself not only depends on the physical properties of a material used for a film forming composition but also depends on the behaviors of the material during film formation. According to the report (for example, Japanese Patent Provisional Publication No. 2005-216895) by the present inventors, a high strength film can be formed by the steps of: modifying a surface of a silica sol or zeolite particles with a crosslinking group having a high crosslinkability between particles or between a particle and a silicon-oxide-based resin to be added simultaneously; temporarily losing the crosslinkability with a protective means for preventing the crosslink formation or deactivation of the crosslinking groups during stable storage; and sintering after application for removing the protective means and developing the high crosslinkability again.